Luminescence quenching of tris(2,2'-bipyridine)ruthenium(II) by 2,6-dimethylphenol and 4-bromo-2,6-dimethylphenol in sol-gel-processed silicate thin films.

The luminescence properties of tris(1,2-bipyridine)ruthenium(II) (Ru(bpy)(3)(2+)), included in different organically modified silicate gel matrixes were investigated. Spin and dip-coated thin films were prepared from methyltrimethoxysilane (MTMOS) and methyltriethoxysilane (MTEOS). A blue shift in the emission spectrum of the MLCT excited state of Ru(bpy)(3)(2+) with respect to the aqueous solution was observed in all the films, practically independent of the reaction pH used to prepare the "sol," silane-derived precursor, and procedure used (dip-coating or spin-coating) to obtain the film. A bimodal distribution of probe sites in the films was obtained from modeling of the emission decays by a double exponential and from application of the exponential series method. The parameters of the decay components depended principally on the thermal treatment used in the processing of the films. The lifetimes decreased with the increase in the drying temperature of the films; at the same time, the emission spectra showed a red shift and the luminescence efficiency decreased. A luminescence quenching of the ruthenium complex in the films by 4-bromo-2,6-dimethylphenol and 2,6-dimethylphenol in aerated aqueous solution at pH 12 in contact with the film was also observed. The quenching plots obtained from luminescence intensities or luminescence intensity decay measurements showed a downward curvature. These plots could be fitted satisfactorily by a sum of two Stern-Volmer terms with quenching constants K(SV1) and K(SV2) associated with two different binding sites of the ruthenium complex. This result is indicative of the matrix microheterogeneity in the films and is fully consistent with the biexponential nature of the luminescence intensity decay profiles. The Stern-Volmer parameter values for both sites in the films suggest that only a low percentage of the probe is accessible to the quencher and its respective constant K(SV1) is lower than in water.

Luminescent properties of tris(2,2'-bipyridine)ruthenium(II) in sol-gel-processed dip-coated thin films.

The luminescence decay and spectral behavior of ruthenium(II)-tris-1,2-bipyridine dichloride dissolved in different organically modified silicate gel matrixes were investigated. Dip-coated thin films were synthesized from tetraethoxysilane (TEOS), methyltriethoxysilane (MTEOS), ethyltriethoxysilane (ETEOS), and methyl- trimethoxysilane (MTMOS). A blue shift in the ruthenium complex emission spectrum with respect to the aqueous solution was observed for all the films on the sol to gel conversion. This spectral shift was slightly dependent on the precursor used to obtain the films and independent of the reaction pH to prepare the "sol". In the data treatment of the time-resolved luminescence measurements, it was assumed that the distribution of the luminophore in the films was nonhomogeneous. The analysis of the luminescence decay profiles was based on a multisite model. All decay curves are best described by a double-exponential model. The parameters of the decay components depended principally on the thermal treatment used in the processing of the films. The lifetimes decreased and the emission espectra showed a red shift with the increase in the drying temperature. A luminescence quenching of the ruthenium complex in the films by dissolved oxygen in aqueous solution was also observed. The quenching rate constant obtained from the preexponential amplitude-weighted mean lifetimes (tau(M)) was in the order of 10(9) M(-1) s(-1). When a phenolic derivative was used as quencher the process rate was greatly reduced compared to the quenching in water. It would seem that the metallic complex sequestered within the film is placed either into a higher microviscosity microenvironment or in a location which the phenolic quencher cannot access. In both cases, the quenching plot based on tau(o)(M)/tau(M) could be fitted satisfactorily by a sum of two terms of Stern-Volmer. This fact is indicative of the matrix microheterogeneity for the films and is fully consistent with the biexponential nature of the luminescence intensity decay profiles.